The present invention relates to the structure of a wire for a wire electrode cutting an electrode workpiece by electro-erosive intermittent electrical discharges, the wire having a core made of electrically conductive material and a coating of another material having a vaporization and melting temperature lower than that of the material forming the core. There is a category of EDM apparatus utilizing an electrode tool made of a metallic wire connected to an electrical pulse generator such as to cause electrical discharges to occur between the wire and a workpiece for cutting the workpiece. Wire-guiding and feeding means providing substantially rapid longitudinal motion of the wire form part of the EDM apparatus such that the active portion of the wire is continuously renewed, the guiding means providing also a lateral feed of the wire into the workpiece at a much slower rate for effectuating a cut in the workpiece according to a desired path. A small diameter wire, for example of a few tenths of a millimeter, permits to cut very fine details in the workpiece, but presents the disadvantage of considerably limiting the current intensity flowing through wire and the force of tension applied to the wire if rupture of the wire is to be avoided. Due to the lack of rigidity of the wire, precision and efficiency of machining substantially suffer. As the width of the gap between the wire and the workpiece is but a few microns, the smallest disturbing forces applied to the wire causes accidental engagements of the wire with the workpiece surface, which generally simply creates short circuits during which there is no machining of the workpiece.
Various attempts have been made in the past to improve wire electrodes. Copper, which was in common use at first, has been replaced by brass. Various alloys having improved mechanical strength under traction have been tried. Wire consisting of a steel core coated with a metal having good electrical conductance have also been tried. In United States Patent application Ser. No. 14,927, filed Feb. 27, 1979, and assigned to the same assignee as the present application, there is disclosed an electrode wire coated with a metal or alloy having a low vaporization temperature, such as zinc, cadmium, tin, lead, antimony, bismuth and alloys thereof, protecting the core of the wire against thermal shock resulting from the occurrence of electrical discharges and which permits to increase the frequency of the electrical discharges without running the risk of rupturing the wire. Some of the best results have been achieved with a zinc coating which, in a list of elements classified according to their vaporizing temperatures, is the first metal in the list presenting little, if any, inconveniences such as, for example, intense chemical activity or toxicity. The fact that zinc is easily electro-eroded does not present any inconvenience, because the portion of the wire subjected to electro-erosion is constantly renewed during cutting of the workpiece by electrical discharges.